Procedure for the continuous production of free flowing and homogenous mixtures of plastic materials with admixtures

ABSTRACT

Processes for continuous conversion of granulated PVC material and additive substances to free-flowing pre-blends mechanically intermix at room temperature granulated PVC material and additive substances in a first zone continuously transversed thereby to provide a macroscopically homogeneous admixture thereof. The additive substances are melted in a subsequent second zone and the molten additive substances are absorbed with the granulated PVC material in a subsequent third zone to provide the free-flowing pre-blend. This pre-blend advantageously may be passed through a second set of three zones corresponding to the mentioned first, second and third zones.

This is a continuation-in-part of our copending patent application Ser.No. 06/642,917, filed Aug. 6, 1984, now abandoned, for Procedure for theContinuous Production of Free Flowing and Homogeneous Mixtures ofPlastic Materials with Admixtures, and assigned to BUSS AG, ofSwitzerland.

The subject invention relates to a process for continuously convertingPVC and additives, possibly with fillers, to a free flowing plasticspre-blend, and

refers to a procedure for the dispersion of granular or pulverizedsolids with liquid components, in particular for the continuousproduction of PVC-Dry-Blend in a vertical continuous blender providedwith centrifugal disks rotating from a vertical shaft.

It is the task of the invention to propose a procedure capable ofproducing reactive and free flowing powders or agglomerates by means ofblending and wetting granular or pulverized solids with liquids. Inorder to reduce the thermic stress to a minimum during the blendingprocess and by maintaining the blending quality, that is, because eitherthe solid particles and/or the liquid component are sensitive to aprolonged thermic exposure, the blending time is to be reduced to a fewseconds.

This process enables a confectioning of granulated PVC materials in thesense of stabilizing the PVC in such a manner that it subsequentlyresists the high requirements at elevated temperatures in a kneadingmachine or other further processing apparatus. In this respect, thedecomposition temperature is lower than the further processingtemperature, so that the decomposition temperature need to be increased.This enables the prepared product to be processed further at hightemperatures, without being decomposed thereby.

According to prevalent opinions, during a "dry blend", a PVC-Hot-Mixturemust reach a blending temperature that is below the plasticization rangeof a PVC-mixture, e.g., depending on the preparation procedure, between85° and 150° C.

For plasticized PVC a softening agent is added at 50° to 60° C. Forunplasticized PVC the plastic raw material will be heated to the pointof softening or melting in order to agglomerate with the admixedstabilizers and possibly fillers and to form a free flowing sinteredpowder. According to prior art, for this level of preparation, machinesthat proceed step by step are applied. They are either slow running andheatable ribbon flights or plough bar blenders or fast runningturbomixers with a small chamber volume whereby a hot mixer isfrequently combined, with a subsequent cooling mixer, in order toquickly cool the batch. Since the temperature increase depends on thesize of the batch, the specific temperature of the product and theintroduction of energy per time unit and since the first two parametersare set, the developments always concentrated upon introducing higherlevels of energy per time unit into the blend.

During the hot mixing or dry blend, that is blending and simultaneousheating of the PVC to just below the plasticization temperature, that isapprox. 130° C., all additives are ground up, melted down and absorbedby the PVC-granulate. The product of the hot mixing process is called"dry blend". In order to produce "dry blend", energy introductions of100-150 KJ/kg are deemed necessary.

Other plastic pre-blends are prepared in a similar manner.

Surprisingly, it has now been discovered that a high blending qualitycan be achieved even by a most gentle preparation process with minimalenergy introduction, if the solids are vortexed at high revolutions andcontinuously dosaged into a continuous blender at room temperature andif the solids are simultaneously wetted by a highly heated liquidcomponent. Hereby it is useful if the liquid component is heated acutelyand immediately before dispersion.

For plasticized PVC it is advantageous if the liquid component is heatedto 80°-200° C. and blown in. For unplasticized PVC, the passage slitbetween the inner wall of the housing and the continuous mixer and therotors in form of centrifugal disks, is made smaller in an essentiallyknown manner and high energies can thus be introduced kinematically intothe penetrating material. The additives melt within a fraction of asecond thus blending with the added solid admixtures. Cooling takesplace in the process and a free flowing powder is thus formed.

The momentary thermic stress is advantageously short due to the veryshort dwelling or residence time. The procedure is simple, low in costsand fast, resulting in a high quality blend.

With regard to the definition of residence time, the following should befurther explained. In principle, the result of a continuous mixingdepends to a decisive extent on the introduction of the individualcomponents within the time unit. The more constant the individualcomponents are added within the time unit, the easier the task for thecontinouus mixer. The border line case of a continuous mixer requires atheoretical volume near zero, namely then when the dosage could becontrolled so precisely, that, according to the required blendingproportions, the particles of the components are brought togetherindividually. Since this is not practically possible, blending times arenecessary similarly as in batch mixers. Instead of using the termblending time one speaks now of residence time. For the achievement of ahigh blending quality, a residence time as short as possible isendeavoured. However, this short residence time in the mixing area doesnot allow for sufficient heat supply by means of heating the mixingtools and walls which was considered indispensable up to now.Furthermore, the mechanical stress of the particles was insufficientduring the blending process.

All of the above problems and disadvantages are being solvedadvantageously according to the procedures of the subject invention. Therequired quality of the blend is easily attained by means of theproposed heating of the liquid component. In case the liquid componentreacts sensitively to a temporary thermic stress, the heating takesplace in an instantly effective continuous flow heater immediatelybefore injection.

According to an aspect of the subject invention, a process for acontinuous conversion of granulated PVC material and additive substancesto a free-flowing pre-blend, comprises, in combination, the steps ofmechanically intermixing at room temperature the PVC material andadditive substances or starting materials in a first zone continuouslytransversed thereby to provide a macroscopically homogeneous admixturethereof, melting the additive substances in a subsequent second zone,and absorbing the molten additive substances with the granulated PVCmaterial in a subsequent third zone, so that granulated PVC having amomentarily stickly surface occurs, to which filler, if present,adheres. It is thereby advantageous to vortex the starting materials inthe first zone mechanically and to heat them by mechanical frictionmomentarily in the second zone. Mechanical vortexing of the startingmaterials may be effected by means of a rotating disc mixer at highrevolutions, wherein the starting materials subsequently are driventhrough a narrow gap in which a melting of the additives takes place.

It is further useful to wet the macroscopically homogeneous admixture ofPVC material and additive substances or starting materials additionallywith at least one heated liquid component.

It is also advantageous to pass the pre-blend through a second set ofthree zones corresponding to the above mentioned first, second and thirdzones.

Apparatus for the performance of the proposed procedure are shown by wayof example in the accompanying drawings in which:

FIG. 1 is a schematic drawing of the apparatus,

FIG. 2 is a vertical section of the blending device of the apparatus,

FIG. 3 is a plan view of the housing of the blending device,

FIG. 4 is an enlarged presentation of a rotor, partially in verticalsection, and

FIG. 5 is a top view of the rotor.

FIG. 6 is a variant of FIG. 1.

An apparatus as cited by means of example in FIG. 1 is being used forthe dispersion of granular or pulverized solids with liquid components,particularly in the continuous production of plasticized PVC Dry-Blend.The apparatus according to FIG. 6 serves in the preparation of freeflowing unplasticized PVC pre-blends.

As evident in FIG. 1, the schematically presented apparatus is providedwith a container 2 with a heating jacket and a stirring apparatus forthe acceptance of the liquid components and is connected by means of apipe 3 to a vertical continuous mixer 13 which is provided with acylindrical housing 5.

The housing length corresponds to approximately the doubled diameter ofthe housing. A container 2' is switchable to the pipe 3 and serves forthe acceptance of a rinsing liquid for the purpose of cleaning thecontinuous mixer 13 when changing liquid components.

The inside of the pipe 3 is provided with a dosing or spinning pump 6, aretroheater 37 and a flowmeter 7. The pipe 3 mentioned above isconnected to an annular pipe 8 surrounding the housing 5 and providedwith injection nozzles 9 leading into the inner side of the housing 5through which the liquid components will be injected from the container2 into the continuous mixer 13.

A balance silo 10 which is provided on its floor with a discharge screw11 which leads into the housing 5 of the continuous mixer 13 serves forthe acceptance of solids. The outlet of the discharge screw 11 islocated somewhat above the annular pipe 8 in the housing 5. The silo 10as well as the discharge screw 11 are arranged on a platform balance 12.The discharge screw 11 is driven by the regulating motor M₁. Thevertically arranged continuous mixer 13 is provided with a verticalshaft 14 extending downwards, equipped with tworotors 15. The shaft 14is driven by a motor M₂ arranged on top. The detailed embodiment of thecontinuous mixer 13 can be seen in FIG. 2-5. The housing 5 of thecontinuous mixer is provided with bearings 28 in which the shaft 14 isoverhung. The upper end of the shaft 14 extends from the housing 5 andis provided with a pulley 29 which is in connection by means of adriving belt 30 with the motor M₂ not presented here. The lower end 31of the shaft 14 is tapered and is equipped with two rotors 15 tightlyarranged above each others. In the area of the rotors 15 the wall 39 ofthe housing 5 is smooth and above the upper rotor 15 an annularcontraction 32 is provided where one to three injection nozzles 9 forthe liquid components are arranged. The entire cylindrical housing 5 isparted and can be folded out; it is suspended from a triangular mountingplate 33 and held together by means of clamping screws. The mountingplate 33 is resting on mounting buffers 33' and the housing 5 is held bytwo columns 34 and a rocking shaft 35. For the purpose of supplyingsolid pre-blends, the discharge screw 11 is connected to the nozzle 36on the housing 5.

The shape of the rotor disks 15 can be seen in FIG. 4 and 5. They areprovided with radially distributed wings 38. However, other suitablerotor forms can be used whereby they are adjustable to a tangentialvelocity of 3-50 m/sec. An essential reduction of mean residence time,preferably to 2-5 seconds, of the product between the rotors, that isbetween the lower rotor and the outlet, could be achieved by means ofthe above described simple arrangement of the rotors and the smoothhousing walls to the effect, that a sticking of the parts of the mixingaggregate, which are in contact with the product, is avoided. The highrevolution of the shaft, at preferably 2000-7000 rpm and theconstructive execution of the mixing aggregate allow an unimpededcontinuous production of high quality solid dispersions at a minimalexpenditure of energy which is only approx. 0.01-0.08 kWh/kg. The axiallength of the mixing zone approximately corresponds with the diameter ofthe housing 5 of the device. The solid supply and the dry soliddisintegration require the same length. From this results the totallength of the procedural part in the housing which corresponds with thedoubled diameter.

Due to the mounting described above, the shaft can be suspended oroverhung whereby the bearing on the discharge end can be eliminated.This permits a free and unobstructed discharge from the mixing aggregatewhich is so important in the production of dispersions. The product canbe degassified by applying a vacuum in a downpipe immediately after thedischarge without necessitating any further heat supply. This is a veryeconomical solution.

If the blend is not provided with a substantial proportion of a liquidcomponent, e.g. if a free flowing and homogenous blend for unplasticizedPVC is produced, then the liquid component must be producedkinematically in the form of softening and melting of the admixedadditive portion by means of vortexing. Hereby equal dispersion of alladditives is more critical and more difficult than with flexible blends.

The apparatus according to FIG. 6 has been adjusted to theserequirements. The continuous mixer 13 is the same as in FIG. 1, however,without the injection nozzles.

PVC and filler and fed separately to the housing 5 at 40 and 43,respectively, with the aid of a volumetric or gravimetric feeders 11,and both go into the feed-through mixer 13 where the rotors 15 arelocated. Additives may be added at 44.

In order to narrow the passage slit 46 between the wall of the housing 5and the rotor, rings 45 are clamped to the rotors 15. In order to beable to preselect the width of the slit clamping rings of variousthickness are available.

Variations of shearing slit and revolution allow an adjustment of theenergy to be introduced with respect to the quality specifications insuch a manner that the temperature/time stress is kept to a minimallevel.

Each of the rotor discs 15 and adjacent parts of the housing 5 establishzones for processing the materials fed through the apparatus 13. Inparticular, the starting materials are mechanically intermixed at roomtemperature in a first zone continuously transversed thereby, so that amacroscopically homogeneous admixture thereof is formed. That first zoneis located immediately ahead of the top rotor 15, and there is such afirst zone ahead of the or each subsequent rotor 15.

Melting of the additives then takes place in a subsequent second zone,which encompasses each rotor 15 between its periphery and the housing 5at the narrow gap or shearing slit formed at wings 38.

The molten additives are absorbed by the granulated PVC in a subsequentthird zone, so that granulated PVC having a momentarily sticky surfaceoccurs, to which filler, if present, adheres. That third zone is thespace or area located immediately after the top rotor 15.

It is thereby advantageous to vortex the starting materials in the firstzone mechanically and to heat them by mechanical friction momentarily inthe second zone. Mechanical vortexing of the staring materials may beeffected by means of a disc mixer having rotor means 15 rotating at highrevolutions, and these materials are subsequently driven through thenarrow gap or second zone in which a melting of the additives takesplace.

It is further useful to wet the macroscopically homogeneous admixture ofstarting materials additionally with highly heated liquid components.

According to the illustrated preferred embodiment of the invention, theprocessing apparatus 13 has disc-shaped mixing devices 15 arranged, oneafter another, on a shaft 14 situated in a closed vertical housing 5,and delimiting with the inside wall of that housing the gap or secondzone in which the melting of the additives takes place.

Because the materials after admixture in the first zone are driventhrough a very narrow gap or second zone, a temperature increase occurssuddenly and locally limited, whereby the additives are melted andabsorbed by the PVC granules. On the sticky or tacky surface of the PVCgranules, the filler substances become stuck or adhere. This effectivelysolves a problem of prior-art approaches, since absorption of additivesonly takes place when they have a sufficient temperature. If theadditives were too cold, a short transit time would be insufficient forabsorption thereof by the PVC, in the absence of the narrow gap orsecond zone according to the subject invention.

The above mentioned first, second and third zones are repeated with theor each subsequent rotor 15, whereby each of the rotors 15 has a firstzone located immediately ahead thereof, a second zone therearound, and athird zone immediately thereafter.

The proposed process may be employed for plasticized, as well asunplasticized or rigid PVC. It is thereby important that the temperatureof dry blend is higher than 60°-80° C., since the additives are notabsorbed at lower temperatures.

A confectioning of granulated PVC takes place in the sense that with thepreparation according to the subject process, the PVC is so stabilizedthat it resists the higher requirements of subsequent processing in akneading machine or other processing apparatus at high temperature.

The PVC blends made according to the subject invention are useful in themanufacture of practically all commercially manufactured and sold PVCarticles, including PVC window frames or profile stock, PVC sheets, PVCtubing, and many more, and may be employed in extrusion, injectionmolding and other processes used in forming PVC articles.

An outstanding advantage of the subject invention is that themanufacturing process takes place continuously, rather than in stages,as in the prior art. In practice, this enables a one-stepcontinuous-feed operation of the continuously running equipment, withoutthe formerly customary step-by-step procedure, with intermediate storageof materials.

PVC dry blends produced according to the subject invention may be feddirectly and continuously to kneading, extruding or molding equipment.This results in a substantial increase of throughput and production andproduct quality, at energy savings of from 10% to 50%.

Customarily, dry blend had to be cooled after its preparation in aseparate cooling mixer. The subject invention dispenses with thisrequirement, since it enables the preparation of PVC dry blend atconsiderably lower temperatures than before.

Applications cited by way of example:

1. In a mixing apparatus; as described above in FIG. 1 as an exemplifiedembodiment, with a housing diameter of 100 mm and equipped with a dosingpiston pump for the liquid component and a weighing belt for the solidpreblend, and with the shaft of the mixing apparatus turning at 7000rpm, liquid components were introduced at a temperature of 180° C. andsolids at room temperature and 200-300 kg/h of dispersed powder wasproduced in continuous operation and, if necessary subsequentlyimmediately deareated by means of a vacuum. Three injection nozzles wereapplied and two rotor disks arranged at the lower part of the shaft. Dueto the very short residence time, the increased shorter wetting speedcould be taken into account thus allowing for an unobstructed dischargeof the dispersed product.

In this example, the liquid component may comprise a plasticizer, suchas dioctylphtalate (DOP), diisooctylphtalate (DIOP), or tricresylphosphate. Such plasticizer may be present in an amount of about 20% byweight of the liquid and solid pre-blends.

Solid pre-blends may comprise about 50% by weight of PVC and 5% byweight of additives, as well as filler. Suspension-type PVC with aK-factor or value of 57, 60 or 80 may be employed. Suitable additivesinclude lead or organic stabilizers or polyethylene wax. Suitablefillers include chalk, calcium carbonate, titanium dioxyde or talcum.

2. The same arrangement as in the first example is used for thecontinuous homogenous preparation of plasticized PVC cable mass. In thevortex chamber a hot liquid component consisting of a mixture of 41parts of diisooctylphthalat (DIOP), as a softener and 20 parts ofchlorinated paraffin, 52% Cl, as an extender, is injected to 100 partsof suspension-PVC with a K-factor of 65, which was coarsely pre-blendedwith 70 parts of ground and surface treated calcium carbonate as afiller and 4.7 parts of tribasic lead(II) sulphate as a stabilizer. Thecontinuous production yields a homogenous dry-blend that is stable instorage.

3. Application cited by way of example according to FIG. 6: A continuousmixer with a housing diameter of 100 mm as described above was fed withPVC, filler and additives from three metering scales. These three solidparts were intensively vortexed and mixed and forced through two narrowshearing slits of 0.6 mm between the rotor and the housing wall. Therevolutions of the rotor were 5000 rpm. The particles of the resultingfree flowing and homogenous pre-blend were discharged at 95°-115° C.from the continuous mixer after a short mean residence time. The productcould be stored without a special cooling step without it setting up.The throughput was 200 Kg/hr.

Bulk or suspension-type PVC may be employed in this example. By way ofexample, suspension PVC with a K-factor of 70 may be employed with chalkas a filler. Such PVC may, for instance, be present in an amount of 70%,with 25% filler, and 5% additives, such as 2.5% stabilizers, 1.8%lubricant and 0.7% color pigment. Suitable stabilizers include cadmiumzincate, and suitable lubricants include stearic acid.

According to a further example, 90% of bulk PVC of a K-factor of 70 maybe blended with 4% of carbon black as filler, 4% of mercaptan-typestabilizer as additive, and 2% lubricant, also as an additive.

According to still another example, 65% of suspension-type PVC of aK-factor of 65 and 20% of PVC acryl copolymer are blended with 10% oftitanium dioxyde as filler, 3.5% of a zinc-type stabilizer, and 1.5% ofa lubricant, such as E-wax.

4. Application cited by way of example as above, an unplasticizedPVC-plastic pre-blend for bottles was continuously prepared according tothe following formulation: 100 parts of a bulk or suspension-PVC (40)with a K-factor of 50-55 are continuously fed into the inlet of thecontinuous mixer (13) by means of a metering scale (12), simultaneouslyanother metering scale feeds the additive of a pre-blend of 10 parts ofmethylmetacrylate-butadiene-styrene copolymer (MBS) for impact strengthand 1.2 parts of thio-dioctyl stannate as a stabilizer and a thirdmetering scale feeds 1.5 parts of calcium stearate as a processingagent. If the preheating of the softening agents is simultaneouslyapplied in combination with a narrowed shearing slit, the degree ofeffectiveness of the kinematic energy-introduction is 60-70% withunplasticized PVC, resp. 45-70% with plasticized PVC. Examples 2 and 4are based on formulations as cited in the following publication:

BHL-Compounding Manual by W. V. Titow et al. Published by Buss-HamiltonLtd. (1979), Chapter CA1, p. 6, and CA3, p. 3.

The subject invention resides also in apparatus for a continuousconversion of granulated PVC material and additive substances to afree-flowing pre-blend, comprising a rotary disc mixer, such as themixer 13 shown in the drawings and described above as forming with theaid of one or more rotors 15, rings 45 and wings 38, the defined first,second and third zones, with sufficiently narrow gap 46 to effectmelting of additive substances.

The subject extensive disclosure suggests to those skilled in the artvarious modifications and variations within the spirit and scope of thesubject invention and equivalents thereof.

We claim:
 1. Process for a continuous conversion of granulated PVCmaterial and additive substances to a free-flowing pre-blend, comprisingin combination the steps of:mechanically intermixing at room temperaturethe PVC material and additive substances in a first zone continuouslytransversed thereby to provide a macroscopically homogeneous admixturethereof; melting said additive substances in a subsequent second zone;and absorbing the molten additive substances with said granulated PVCmaterial in a subsequent third zone to provide said free-flowingpre-blend.
 2. Process according to claim 1, including the stepof:passing said pre-blend through a second set of three zonescorresponding to said first, second and third zones.
 3. Processaccording to claim 1, including the step of:wetting said macroscopicallyhomogeneous admixture of PVC material and additive substances with atleast one heated liquid component.
 4. Process according to claim 3,including the step of:passing said pre-blend through a second set ofthree zones corresponding to said first, second and third zones. 5.Process according to claim 1, wherein:said mechanically intermixed PVCmaterial and additive substances are momentarily heated by mechanicalfriction in said second zone to melt said additive substances. 6.Process according to claim 1, including the step of:driving saidmechanically intermixed PVC material and additive substances through agap sufficiently narrow to effect said melting of the additivesubstances.
 7. Process according to claim 1, including the stepsof:providing a rotating disc mixer for mechanically intermixing said PVCmaterial and additive substances in said first zone and driving saidmechanically intermixed PVC material and additive substances through agap in said second zone sufficiently narrow to effect said melting ofthe additive substances.
 8. Process according to claim 1, including thesteps of:driving said mechanically intermixed PVC material and additivesubstances through a gap sufficiently narrow to effect said melting ofthe additive substances; and passing said pre-blend through a second setof three zones corresponding to said first, second and third zones. 9.Process according to claim 1, including the steps of:driving saidmechanically intermixed PVC material and additive substances through agap sufficiently narrow to effect said melting of the additivesubstances; wetting said macroscopically homogeneous admixture of PVCmaterial and additive substances with at least one heated liquidcomponent; and passing said pre-blend through a second set of threezones corresponding to said first, second and third zones.
 10. Processfor a continuous conversion of granulated PVC material, additive andfiller to a free flowing preblend, comprising in combination the stepsof:mechanically intermixing at room temperature the PVC material,additives and filler in a first zone continuously transversed thereby toprovide a macroscopically homogeneous admixture thereof; melting saidadditives in a subsequent second zone; and absorbing the moltenadditives with said granulated PVC material in a subsequent third zoneto provide said PVC material momentarily with a sticky surfact to whichsaid filler adheres, to provide said free-flowing pre-blend.
 11. Processaccording to claim 10, including the step of:passing said pre-blendthrough a second set of three zones corresponding to said first, secondand third zones.
 12. Process according to claim 10, including the stepof:wetting said macroscopically homogeneous admixture of PVC material,additives and filler with at least one heated liquid component. 13.Process according to claim 12, including the step of:passing saidpre-blend through a second set of three zones corresponding to saidfirst, second and third zones.
 14. Process according to claim 10,wherein:said mechanically intermixed PVC material, additives and fillerare momentarily heated by mechanical friction in said second zone tomelt said additive substances.
 15. Process according to claim 10,including the step of:driving said mechanically intermixed PVC material,additives and filler through a gap sufficiently narrow to effect saidmelting of the additive substances.
 16. Process according to claim 10,including the steps of:driving said mechanically intermixed PVCmaterial, additives and filler through a gap sufficiently narrow toeffect said melting of the additive substances; and passing saidpre-blend through a second set of three zones corresponding to saidfirst, second and third zones.